In the field of processing materials with ion beams, various techniques have been developed for producing large, approximately-parallel ribbon ion beams with controlled current uniformity. In this context, the term ‘controlled’ is understood to mean that the current density along the long transverse dimension (the target direction or travel axis) of the beam adheres to a desired profile, which may be uniform (i.e., homogeneous, symmetrical, or regular) or may be variable and non-uniform (i.e., heterogeneous, asymmetrical, or irregular) in a predetermined manner or a pre-chosen pattern (such as a left-right linear ramp).
Examples of ion implanters which employ a continuous ribbon beam, but which omit any active means of controlling the uniformity, can be found. Many of these also omit any means of analyzing the ion beam so as to remove contaminant species. For examples, see Armini et al., “Non-Mass-Analyzed Solar Cell Ion Implanter”, in Nucl. Instr. and Meth, B6 (1985), p. 94, Elsevier, North Holland.
Among the conventionally available assemblies and techniques for controlling the current density uniformity of ion beams are the following:
(i) The invention disclosed by U.S. Pat. No. 5,350,926 teaches the use of magnets for analyzing, shaping and rendering parallel an ion beam as well as the use of multipole elements (either integrated into bending magnets or as separate assemblies) for controlling the uniformity of the beam.
(ii) In a commercial implantation system sold by Varian Semiconductor Equipment Associates Inc. (known as the “VIISta-80 ion implanter”), the physical movement of members of a discrete set of pole pieces within a deflection magnet produces variations in a local dipole field component lying normal perpendicular) to the long dimension of the ribbon beam and to its direction of travel.
(iii) In a commercial implanter sold by Mitsui Engineering and Shipbuilding (the MDI-100), a discrete multipole device is presented as a rectangular array of iron pole pieces mounted on a yoke which surrounds the ion beam. Each pole piece is individually excited by a separate coil wound around it. The resulting magnetic field is applied in the central rectangular aperture, through which the ribbon-shaped ion beam passes; and consists of spatially varying dipole components, which cause a local slight deflection of the trajectories for the ions passing through it. Subsequently, at a processing plane downstream from the multipole device, the trajectory deflections produce a characteristic variation in the current density for the ion beam, in which one region typically exhibits a decrease in ion density while a neighboring region exhibits an increase in ion density. See for example, U.S. Pat. Nos. 5,834,786 and 5,350,926 for additional details of this arrangement.
(iv) Algorithms for adjusting multipole devices to achieve a greater degree of current density uniformity have been developed by Diamond Semiconductor Group Inc. and are typically used in the manufacture of their commercial products. However, such algorithms are very complicated in their specifics; and are quite difficult to implement in practice as a functional part of an ion implantation system.
(v) One conventionally known format of a multipole lens [e.g., Banford, in The Transport of Charged Particle Beams, Spon, 1960] is shown by Prior Art FIG. 1. As seen therein, the multipole lens is conceived with rotational symmetry. The magnetic field generated therein can be expressed in terms of cylindrical harmonics, and is best described using a polar coordinate system. Such lenses are used in various applications of generally cylindrical ion beams, such as electron microscopes and accelerators, where they can control aberrations of the system optics.
(vi) Attention should also be given to the “Panofsky” quadrupole lens design described by Banford [in The Transport of Charged Particle Beams, Spon, 1960] and illustrated by Prior Art FIG. 2. This multipole format uses a closed rectangular yoke of iron to make a quadrupole lens for a beam of high aspect ratio. The windings on the two long member pieces of the yoke, which extend in one direction, must carry the same ampere turns (but in the opposite sense) to the two windings on the short member pieces that close the yoke and are oriented in the other direction. Both pairs of windings must be uniform in cross section in order to generate a uniform field gradient within the central region. The windings on oppositely positioned sides of the yoke are electrically excited to yield a zone of linearly varying magnetic field, i.e. dBy/dx=−dBx/dy, which is approximately constant within the space bounded by the coils.
(vii) Another previously known format is the “Cartesian” multipole lens of White et al. [disclosed in the IIT '98 conference published by IEEE] which conforms to the shape of a ribbon beam, and is illustrated by Prior Art FIGS. 3 and 4 respectively. The device (shown in cross-sectional view by FIG. 3 and in a detailed sectional view by FIG. 4) is a rectangular multipole lens which conforms to the shape of a ribbon beam in order to control its uniformity; and is often referred to as a Cartesian multipole—since it is best described in Cartesian coordinates, rather than by polar coordinates. Accordingly, this multipole lens produces a field component “By” whose variation along the x-axis can be controlled directly, by varying the current of the coils at different x-coordinates, with a resolution determined by the pitch of the coils and poles. Prior Art FIG. 5 shows the effect of exciting a single pair of coils within this “Cartesian” multipole on an otherwise uniform ion beam.
In most types of systems using continuous ribbon beams, provision is made to move the workpiece to be implanted through the ion beam, in the direction of its short dimension, at a controlled velocity effective to achieve the correct dose of ions. In some systems a single passage is used, and in others each workpiece moves multiple times through the ion beams. The advantage offered by this technique is that minor beam size fluctuations in the y-axis direction have no net effect on the uniformity of the processing.
Overall therefore, many of these previously known structures and conventional ion implantation systems have been commercially utilized; have been technically successful in some meaningful degree; and have been reported within the technical literature with complete descriptions of their use and manner of operation. It is noteworthy, however, that the multipole structures within all these known systems all have been designed to provide a magnetic field whose strength is controlled as a function of the x-coordinate/dimension of the flowing ion beam. In addition, they often require involved software algorithms to operate; and, in general, are beyond the competence of a modestly skilled operator to understand fully or to control effectively.
For these reasons, there remains a well recognized and long standing need for the development of an assembly which would provide the user with the capability of adjusting a single operational control in order to raise or lower the current density at will in a single defined zone (relative to that in the remainder of the beam), with minimal complicating side effects. Desirably also, such an improved assembly for adjusting current density uniformity of a continuous ion beam would be simple and intuitive to operate; would be of simplified design and construction; and would markedly reduce the power consumption and heat load of ion implanters generating a ribbon-shaped, charged particle continuous beams.